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Texas Star DX 350 HDV Input SWR

Discussion in 'Amplifiers' started by Shadetree Mechanic, May 31, 2018.

  1. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Hey Guys, I had this at the end of someone's thread so I thought I would start a new one to make it easier to find. The input on my DX 350 had an SWR of around 5:1. I was able to coil up some coax and help it some. The Dial A Watt was useless because it would peg the SWR meter to the right. The solution ended up with replacing C8 with a 468 trim cap.
    https://www.rfparts.com/468.html


     

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    #1 Shadetree Mechanic, May 31, 2018
    Last edited: May 31, 2018

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Here are the guts of the DX350.

    IMG_20180530_103455422.jpg

    C8 is this brownish chicklet looking capacitor.
    IMG_20180530_103603033.jpg
     
  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I made some legs out of paper clip wire. I bent some feet on the end of the paper clip wire to solder to the traces.

    IMG_20180530_112649120.jpg

    Looks right at home.
    IMG_20180530_112637956.jpg
     
    #3 Shadetree Mechanic, May 31, 2018
    Last edited: May 31, 2018
  4. nomadradio

    nomadradio Analog Retentive

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    Rock on! Well done.

    Hope you won't be using it in a dump truck.

    73
     
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  5. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Did you find a value that works better than what they had?

    I mean, once you got it "tuned right" - were you able to swap out a fixed value?

    If I remember correctly - this needed about 330pF or so...but for my scenario

    And the 75 ohm? The one by your finger? I put that on the "free end" of the variable - so it had somewhere to put power placed into the pot, could be spread ACROSS the pot, into the 3rd leg. Lifted the one leg on the side closest to the switches and then rotated it to meet the pots open leg.

    Used 3 150 ohm 2W composition resistors in parallel (stacking) where the 75 ohm was - making it a 50 ohm for SSB mode... SWR was tolerable less than 1 : 1.3 using a PC-122.

    Now if my family would give that darn thing back to me, I'd show you in pics how I did it..

    :+> Andy <+:
     
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  6. Onelasttime

    Onelasttime Well-Known Member

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    You do not want a variable there anyways! They are a weak point in any design and have been known to not only fail freq. but cause a lot of other problem including SWR issues.

    All of the TS amps that have a variable on them usually will have issues if it has actually been used. That is something you never want to see on an amp if you value durability and good SWR.
     
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  7. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I've had them (variable) arrive in open condition, only to find out they used them in radios with clipped limiters and heavy modulation driven into dual-finals. (Read: Hot-rodded radios)

    Seems that the "2watt composition" once you turn it down in ohmic resistance, as if they are increasing power drive, snap open like a fuse once it gets past a certain spot - or rotation on the wiper to composition angle - low ohmic resistance and using a radio with 8+ more of watts into the amp - it doesn't take long to heat it up and "smokes" the pot.

    So, yes pots and or vari-caps, they are weak, if not the weakest link - so I've had to have done-up the technique posted above used in different setups to allow them to drive them harder - even had to insert a spare 39 ohm composition in line with the wiper terminal to lessen the dissipation across the variable.

    It's why I asked about the capacitive effect across the variable...I've used 330~390 pF instead of the 68pF in there.

    But that was a fixed value to shunt more RF off the variable pot. SWR effects were there too, but when you have someone that's running Lincolns, Jackson or SS3900 with full-fledged dual finals - there's a lot of power that has to get dissipated and sent somewhere....

    The 75 ohm reroute to the open tab on the pot and along with the 3X150 (50 ohm) was for those that used SSB - at least they survived their setups.

    At least with some attenuation - you had a better sounding radio along with a longer lasting setup they could run either way - on or off - keeping the amp inline and not have to fuss with the radios' controls and power drive levels too much.

    Just a lot of parts to save a 2watt variable that took nearly a month of your life to get sent to you in the mail - so you ordered 5 pots or so, along with spare components to replenish what the kids needed under the hood to get their jobs done.

    So, no, I didn't want them to keep coming back begging for me to re-fix it again.

    :+> Andy <+:
     
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  8. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I left the trimmer in place so I could play with it more if needed. I have no idea what value it ended up being. The 75 ohm resistor and how you did yours sounds interesting. I will have to study the schematic some more to figure out what is what. The variable pot when turned on makes the SWR completely flat. Otherwise it is around 1.2 so I don't know if that is because the inductance is different in that circuit for the variable.
     
  9. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I don't plan on using the variable, it can be bypassed with the switch on front. It seems like a gimmick anyway. If I want to run less power I will just turn the amp off.
     
  10. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    I will mainly be using this with my President Madison on side band. Would this be too much drive for it?
     
  11. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    This is the fun part, figuring things out and making them do what you want them to do. I appreciate all the help from everyone here, I am learning a ton. If all I had to do was plug it in and hit the key it wouldn't be as much fun. No dump truck lol this will be part of my new little shack. Looking at the construction of this amp, it appears to be hand assembled and some of the stuff doesn't look ideal but I guess it works.
     
  12. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    This is long - and you'll see why...

    I posted a question about finding more bi-polar parts versus the MOSFET - seems that some of the locals here, want to use their "amps" that were designed more for the older Bi-polar versus the MOSFET repair redo's that a lot of the truckstop - radio shops - offer.

    You bring up a good point...

    Are you using a Bi-Polar? Does the Madison have the NFB on the 2166? OR, do you have the older 1306?

    Either configuration - even then - it will need some method of NFB in the Pre-Driver or Driver to help with drive levels and the sheer dynamic range the power envelope will place on the input of the amp.

    It's not the "Carrier" that kills these things (the pots I'm referring to) - its' the power of the envelope and all the spectral artifacts shoved into the coax going to that amps input is what burns these pots up. (Modulation is not excluded - it is the main portion of the problem)

    The problems lies in how well the customer will follow instructions - you can put something together for them and it'll work great right out of the starting gate, but as time progressed - they had "this n' that" when "this happened" and "when I let someone borrow" CRUNCH - ow - that hurts...

    So my "bulletproof" method was to pad - tone down - lessen - restrict - deny (insert favorite DO NOT DO THIS metaphor here) RF from perforating the finals or pots used in their variable setups - you already know from experience they will run hard over - hard over - 360 X2 rotation. So making it more of a mudduck style works for the guys and makes the shop happier from less electrical problems from the "problem childs" {note plural) they have out in the field. So they, aka "children" will turn their radios back up and harass the hell out of each other on the channels they run because of the stresses of doing the jobs they do - with heavy equipment.

    So unless you prepared for the worst, some one was going to get hurt - then you get into those "wars" over it that are at times not unlike - Hatfields and McCoys type of feuds.

    So that's where my mentor, friend, electrical repair guy played upon my insatiable desire to solve for an improper fraction and never leave a potential pot terminal - unterminated. That guy was also the supplier I went to for a lot of things. Fun days - shame that all that is from that time, is now gone, auctioned off - and I'm left with only the memories.

    Protect your stuff - can't do a *amp thing about their stuff.

    So that is why the open terminal in the box got sent to ground - but not directly - it was an operation parameter decision not just arbitrary. Sure - it lessens the drive level - a larger cap was used to absorb the RF as a shunt - and with some radios - like the 2510 or the TS5020 - their high-drive low-level modulation - you needed something to keep the finals from going to their reward, and from pinching up and FM-ing as any radio overdriving an amp will do. Did I save them any more money? Gawd I still have my kneecaps so I guess the blame went directed deflected to someone else. So, I think they enjoyed the rough housing they did, but if their equipment survived - I don't know. I see someone re-worked a section or two but the new owners want it restored to it's original - that's just the radio - who knows what happened to the amps. (none of my business)

    And for your benefit, take the time to review the locations again, you placed two varicaps on two locations - I would have used them on the couplers caps in and out - to see if extra / less capacitance helps with the output matching of the amps own combiners. It's how I found out the 68pF was not enough to divert RF - at least wattage wise. I wanted them to have a good SWR over a larger range of power levels with less worry. I worked under the principle - Once that it got peaked, then work on the input drive levels and hope like h*ll the kids' gonna understand a simple rule of - it lasts longer if you don't over-do it. At least until the next paycheck...

    Also - location - in your photo you use twins, 2 - 51 ohm and cap combos on the NFB between the combiners. I've seen values from 100 ohms to 39 ohms - good to see you keep it in there - many a tech does not - consider yourself a big plus in that category. I had to install them - I used 100ohm 2W and 0.001uF (102/100V) - so to help the kids with envelope - I kept the 75 ohms' - placing the active side to the open tab on the pot, kept the ground side (heat this ground side to allow the solder to liquefy and let you rotate that part without stressing and simply added/replaced where the 75s' active lead went with 150 ohm 2W carbon composition across to the ground leg of the 75 ohm until the SWR went nearly flat on the SSB mode (power dissipation inclusive) It's why I paralleled 3 of these 150 ohm resistors (6W capacity) there - it looked like hell but it worked. While they used it - which they used that button and they considered their amps to be clean (unpinched audio) and the variable, with the understanding that if the amps "cover" got hot, turn that knob down.

    If they didn't - in TS amps - without the conversion I did, it was sure to blow their high-power finals in whatever their radios units they used, by the SWR issues it generates alone.

    Also - remember the 39 ohm I mentioned a post or two earlier - there is a condition on that - it can be installed BEFORE it arrives to the pot, or cap or AFTER - the cap or towards the input combiner/splitter - you can play with that value and insertion location once you see where this is going.

    That's where I'm at now, I depleted my supply - didn't imagine seeing some of the older radios' I did some work to, come back to have seen works' like new drop ins from someone else, then instructed to have it - removed - and returned to stock so they can, what they had back in the earlier times, back.

    It's painful, but good to see them returned to stock portion, which is more of what I did to their stuff in the first place. They wanted/went back to simple tuneup and run with what ya' got stuff versus the newer flashy stuff. Seems the cost of replacement and the attempts to do-away with an extra stage amp - they wanted it (old style) back.

    The pain from me is the effort I put into the audio lines, the coil - the bi-polar parts - all were dumped off into someones ashtray or trash bin with this new crap in it's place and I'm told to return it back to what it was...

    It's not that easy when you're the one the depends on others that supplied locally - and now they are gone - not only to do you have to find a supplier - one you can trust to do business with without getting scalped in the process. And the ones that took your work for whatever reasons, to their benefit - now is dumped onto your lap and you have to do it all over again.

    SIGH.

    Oh well...Hi how are you? What's New?

    :+> Andy <+:
     
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: Texas Star, SWR

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